you,
mere sight and its muscular adjustments account only for the
dimensions of height (up and down) and of breadth (right and left)
while the third or cubic dimension of _depth_ is a highly complex
result of locomotion in which I include prehension. And inasmuch
as we are dealing with _aspects_ and not with _things,_ we have as
yet nothing to do with this _cubic_ or _third dimension,_ but are
confining ourselves to the two dimensions of extension in height and
breadth, which are sufficient for the existence, the identity, or more
correctly the _quiddity,_ of visible shapes.

Such a shape is therefore, primarily, a series of longer or shorter
_extensions,_ given by a separate glance towards, or away from, our
own centre or extremities, and at some definite angle to our own
axis and to the ground on which we stand. But these acts of
extension and orientation cease to be thought of as measured and
orientated, and indeed as accomplished, by ourselves, and are
translated into objective terms whenever our attention is turned
outwards: thus we say that each line is of a given length and
direction, so or so much off the horizontal or vertical.

So far we have established relations only to ourselves. We now
compare the acts of extension one against the other, and we also
measure the adjustment requisite to pass from one to another,
continuing to refer them all to our own axis and centre; in everyday
speech, we perceive that the various lines are _similar_ and
_dissimilar_ in length, direction and orientation. We _compare;_
and comparing we _combine_ them in the unity of our intention:
thought of together they are thought of as belonging together.
Meanwhile the process of such comparison of the relation of each
line with us to the analogous relation to us of its fellows, produces
yet further acts of measurement and comparison. For in going from
one of our lines to another we become aware of the presence
of--how shall I express it?--well of a _nothing_ between them, what we
call _blank space,_ because we experience a _blank_ of the
particular sensations, say red and black, with which we are engaged
in those lines. Between the red and black sensations of the lines we
are looking at, there will be a possibility of other colour sensations,
say the white of the paper, and these white sensations we shall duly
receive, for, except by shutting our eyes, we could not avoid
receiving them. But though received these white sensations wil